(1) Field of the Invention
The present invention relates to an optical waveguide device which has an optical waveguide comprising a region which is formed in a transparent substrate and which has a refractive index higher than that of the transparent substrate. More particularly, the present invention relates to an optical waveguide device which may be suitably adapted for an element for constituting a demultiplexer, a multiplexer, a demultiplexer/multiplexer, a coupler, an optical switch or the like.
(2) Description of the Prior Art
A multiplexer, a demultiplexer or a demultiplexer/multiplexer is an important device in an optical communication system.
A conventional demultiplexer for demultiplexing mixed light comprising light of three or four wavelengths into light components each of a single wavelength generally utilizes an interference filter. A conventional demultiplexer for demultiplexing mixed light of a higher multiplexing degree involving 8 to 10 wavelengths generally utilizes a diffraction grating. This is because a demultiplexer utilizing an interference filter becomes increasingly complex in structure as the degree of multiplexing increases.
A demultiplexer utilizing an interference filter as shown in FIG. 1, for example, is known. In this demultiplexer, a unit demultiplexer element consists of a pair of graded index type lenses 1 whose end faces oppose each other such that an interference filter 2 is interposed therebetween and their central axes are aligned with each other. Each of the lenses has a length which is 1/4 the periodical pitch of light and has a refractive index distribution wherein the refractive index is maximum at the central axis and parabolically decreases toward the outer periphery. A plurality of (a pair of in this case) such unit demultiplexer elements 3A and 3B, respectively, are bonded such that the central axes are offset from each other. An interference filter 2A or 2B of the respective element 3A or 3B reflects light of a specific wavelength .lambda..sub.1 or .lambda..sub.2 and transmits light of other wavelengths.
In a demultiplexer as described above, mixed light having wavelengths of .lambda..sub.1, .lambda..sub.2 and .lambda..sub.3 supplied through a single optical fiber 4A is demultiplexed, and light components respectively having single wavelengths of .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3 can be obtained from optical fibers 4B, 4C and 4D connected to the elements 3A and 3B.
When a demultiplexer for demultiplexing light having three to four different wavelengths has a configuration as described above, since a plurality of cylindrical lens systems must be connected to each other with offset axes, the overall structure becomes complex and assembly is difficult. Furthermore, insrtion loss of such a demultiplexer is relatively high since the end face of the input optical fiber 4A connected to the lens 1 is not a point source and the lens 1 has aberration.
In view of this problem, another conventional demultiplexer as shown in FIG. 2 has also been proposed. In this demultiplexer, a triangular prism base 5 is coupled to one surface 6A of a transparent substrate 6, and a graded index type lens 1 having a length corresponding to 1/4 the periodical pitch of light and an input optical fiber 4A are connected to the base 5. Similar combinations of prism bases 5, graded index type lenses 1, and optical fibers 4B, 4C, 4D and 4E, respectively, are coupled to the surface 6A and the opposing surface 6B of the substrate 6, through interference filters 2B, 2C, 2D and 2E, respectively. The interference filters 2B, 2C, 2D and 2E respectively transmit light having specific corresponding wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4, and reflect light having other wavelengths.
In such a conventional demultiplexer, light incident through the optical fiber 4A can be collimated by the lens 1 so as to become obliquely incident on the substrate 6, and repeatedly transmitted and reflected by the interference filters 2B, 2C, 2D and 2E, so that light components having the respective wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and .lambda..sub.4 can be obtained through the optical fibers 4B, 4C, 4D and 4E, respectively.
However, a demultiplexer having the above configuration also suffers from the problem of a high insertion loss with an increase in the degree of multiplexing as in the case of the demultiplexer shown in FIG. 1. Because, the light beam diverges during propagation within the substrate 6 since the end face of the input optical fiber 4A is not a point source and the lens 1 has aberration.
Still another conventional demultiplexer is also known, as shown in FIG. 3. In this demultiplexer, an optical fiber 7A for receiving mixed light is cut along oblique planes of 45.degree.. Interference filters 2A and 2B for reflecting light having specific wavelengths .lambda..sub.1 and .lambda..sub.2, respectively, are inserted at such oblique planes of the fiber 7A. Optical fibers 7B and 7C for transmitting light components having the wavelengths .lambda..sub.1 and .lambda..sub.2 reflected by the filters 2A and 2B, respectively, are coupled to the side of the fiber 7A.
In this conventional demultiplexer, in order to obtain light having a sharp spectrum distribution at the fibers 7B and 7C, two other interference filters 2C and 2D for allowing transmission of only light components having wavelengths of .lambda..sub.1 and .lambda..sub.2 are additionally arranged at the interfaces between the fiber 7A and the fibers 7B and 7C, respectively.
The insertion loss of this conventional demultiplexer is relatively low when light of wavelength .lambda..sub.3 is transmitted to an optical fiber 7D which is coaxially connected to the input optical fiber 7A and which produces non-reflected light which has not been reflected by the interference filters 2A and 2B. However, when light components having wavelengths of .lambda..sub.1 and .lambda..sub.2 become incident on the optical fibers 7B and 7C which are connected to the side of the fiber 7A, the beams tend to diverge, resulting in a high insertion loss again.
In an optical communication system, a device called an access coupler is also important as a coupler which divides a portion of data from a trunk line, supplies the divided data portion to a terminal or the like for processing thereat, and combines the processed data from the terminal to the data of the trunk line. An access coupler has been proposed wherein an optical waveguide is formed in a transparent substrate of a transparent material such as glass or a plastic.
As a method for fabricating such a coupler by forming an optical waveguide in a transparent substrate, a method as shown in FIG. 4 is known. In the method shown in FIG. 4, a branch angle .theta. is changed while a width W of the optical waveguide is kept constant, so that the ratio of the light outputs from output terminals 8 and 9, that is, PO.sub.2 /PO.sub.1 is changed. When .theta. is set to be 1.degree. or less, for example, we have the ratio PO.sub.2 /PO.sub.1 .apprxeq.1, providing a two-branch coupler. However, the branch ratio to be adopted in an access coupler is as small as 1/5 to 1/20. In order to obtain such a small ratio, the angle .theta. must be increased. When the angle .theta. is increased, the loss at the branch portion is increased, resulting in a higher insertion loss (PO.sub.2 +PO.sub.1)/PI.
In view of this problem, a method as shown in FIG. 5 has also been proposed. In this method, the angle .theta. is decreased to decrease the insertion loss. More specifically, widths W.sub.1 and W.sub.2 at output terminals 8 and 9 of the optical waveguide are rendered to be smaller than a width W.sub.0 at the input side of the waveguide amd a relation W.sub.1 &gt;W.sub.2 is satisfied, so that the output ratio PO.sub.2 /PO.sub.1 is decreased. However, with this method, in the case of a multimode fiber, only a slight change in the connecting position of the input optical fiber results in a significant change in the output ratio PO.sub.2 /PO.sub.1, so that high-precision setting of the output ratio PO.sub.2 /PO.sub.1 is difficult.